I know you need a 75 in various skills to get Elijah to come down to you into the vault. Now lets assume I start Dead Money at level 1 and don't reach the 75 in any of the skills - am I not able to complete the DLC? o_O

No, the skill checks are more or less just flavour so you can lure him down in whatever way you choose; he comes down anyway. There's a similar one in Old World Blues, when the brain asks what you're going to do about the Think Tank and you have a bunch of "witty" one-liners based on your various weapons skills.